Pardon my potential density here. But the handful of
times I've tried, I've never seemed to find a means to
perform a global search of the archives. Rather it
appears a search may only be within the scope of a
given archive volume. This doesn't seem a realistic
limitation so would someone be kind enough to clue
me in?

An external site search indeed works but it is a bit
roundabout and it picks up index pages as well as
actual threads. Not a huge issue, but the omission
of a conventional site search function seems rather
curious.